A hybrid vehicle driven by a motor and an engine connected in series via a clutch is disclosed in Tokkai Hei 9-286245 published by the Japanese Patent Office in 1997. In this vehicle, at low engine rotation speed when the engine efficiency is low and the vehicle is starting under low load, the clutch is released and the vehicle is driven only by the motor. When the vehicle speed has risen to some extent, the clutch is engaged and the power source is changed over from the motor to the engine.